Thermal processing is widely utilized in semiconductor fabrication. An example use of thermal processing is dopant activation. A dopant provided in, for example, polysilicon or a monocrystalline silicon substrate, is exposed to temperatures of from about 900.degree. C. to about 1,000.degree. C. to improve conductivity-enhancing properties of the dopant. Another example use of thermal processing is an anneal, wherein, for example, amorphous silicon is treated at a processing temperature in excess of 1,000.degree. C. to convert the amorphous silicon to polycrystalline silicon. Thermal annealing can also be utilized in treating, for example, metal/silicon interfaces to form metal silicide. Another example use of thermal processing is to melt and reflow materials. For instance, borophosphosilicate glass (BPSG) can be thermally treated at temperatures of from about 800.degree. C. to about 1,100.degree. C. to melt and reflow the BPSG. Such melting and reflow can create a substantially planarized outer surface of the BPSG, which can be desirable for further processing applications.
A particular method of thermal processing is rapid thermal processing (RTP), wherein a material is processed from one temperature to another temperature at a ramp rate of greater than about 20.degree. C./second, and typically greater than about 25.degree. C./second. RTP can be preferred over other forms of thermal processing. For example, thermal processing occurring at temperatures of greater than 900.degree. C. can cause unwanted dopant diffusion. RTP can alleviate such unwanted dopant diffusion by reducing the time during which a semiconductive wafer is exposed to high temperature processing conditions.
RTP is typically conducted by exposing a wafer to a beam of radiant energy. Standard sources of the radiant energy are xenon lamps and mercury lamps. Typically, the radiant energy is in the form of light having a wavelength of from about 0.8 microns to about 2.5 microns. After the exposure to the light, the wafer is allowed to cool by diffusion of heat from the wafer to its surroundings. Wafers typically cool at a rate of at least about 20.degree. C./second by such diffusion.
A difficulty of thermal processing methods generally, and rapid thermal processing methods particularly, is in maintaining temperature uniformity across a wafer surface during the thermal treatment. For instance, during an RTP process a wafer surface is heated by energy transfer from a radiant energy source. Frequently, some portions of the wafer surface will heat more readily than other portions, resulting in non-uniform thermal conditions across the wafer surface. It would be desirable to develop alternative methods of thermal processing wherein such non-uniform thermal conditions could be substantially alleviated.